Crystallographic studies of several hemerythrin derivatives will be carried out to obtain molecular structural information about the liganded and unliganded states of this non-heme iron respiratory protein. Hemerythrin is an oxygen binding protein found in four phyla of marine invertebrates, sipunculids, brachiopods, priapulids and annelids, and correlation of its structure and function with those of hemoglobin, the Fe-heme respiratory protein, and hemocyanin, the Cu oxygen transport protein, will help determine what molecular properties are important for reversible oxygen binding. Comparison of the three molecules should also point out the structural features important for the biological functions each protein carries out. High resolution structure determinations and refinement of met, azidomet, deoxy and oxyhemerythrin will be possible after diffraction intensities are measured with the multiwire area detector at the University of California, San Diego. The binuclear Fe complexes in met and azidomet refined at 1.5 Angstroms resolution will be compared with small molecule model complexes to determine whether the macromolecular ligand distorts the complexes in producing an energetically favorable metalloprotein tertiary structure. Structural studies of deoxy and oxyhemerythrin will determine if the oxygenation process involves large changes in the structure of the metal center or whether it can be simply described as addition of dioxygen to the complex. There is evidence that a bridging oxygen atom might be involved in the oxygenation reaction, so crystallographic studies of both physiological forms will be helpful in determining the reversible dioxygen binding mechanism. Finally, sulfido derivatives of hemerythrin, one with a mixed-valence Fe(III)-Fe(II) state, will be investigated to determine the sulfur binding mode and the structural properties of the semi-met mixed-valence state. The crystallographic studies of hemerythrin will provide structural information bearing on a number of topics dealing with the biology of respiratory proteins. The hemerythrin structures will provide examples of how metals are bound to macromolecules and carry out biological functions, how metalloprotein structures accommodate various reduction-oxidation states, and how binuclear Fe complexes can reversibly bind dioxygen.